Secondary optic for concentrating photovoltaic device

ABSTRACT

A light concentrating optic for use with a photovoltaic device includes a light pipe having a first end portion for receiving light and a second end portion for outputting concentrated light. An optical element is coupled to the light pipe on the first end portion and configured to form an optical interface between the light pipe and the optical element. The optical element includes at least one light transmitting surface configured to redirect incident light entering the optical element to disperse the light to fall incident on side walls of the light pipe to increase homogeneity and intensity of light through the second end portion.

BACKGROUND

1. Technical Field

The present invention relates to secondary optics for photovoltaicdevices, and more particularly to solar concentrators to homogenize anoptical spectrum and intensity of incident solar radiation imaged into alight pipe.

2. Description of the Related Art

Solar concentrators operate by focusing the light on a photovoltaiccell. Employing optical devices to concentrate the light on thephotovoltaic cell enables the concentrator to operate at high powerdensity levels. A Concentrator Photo-Voltaic (CPV) system employs anefficient photovoltaic element to convert concentrated incident lightenergy to electricity. Some CPV systems employ a triple junctionphotovoltaic cell and receiver to accomplish this conversion. The cellconverts the wavelength spectrum of the sun in three wavelength regionsof the cell. Each region of the spectrum is absorbed in a selectedsemiconductor material that efficiently converts the optical power inthat spectrum region into electrical power.

Efficient CPV systems often use two concentration elements for mostefficient light concentration, primary and secondary. The primaryelement typically comprises one of a Fresnel lens and a focusing mirror.The primary element collects the sun light and focuses the light onto animage in the secondary optic through which the light travels to thephotovoltaic cell. The secondary optic may include a light pipe. A lightpipe (or light prism) includes a solid piece of glass having surfaceswith smooth optical quality finishes, also referred to as a non-imagingoptical element. The light pipe further concentrates the imaged lightfrom the primary optic onto an area precisely matching the area of thecell. However, conventional simple light prisms commonly used in theindustry are not optimal in uniformity of intensity and homogeneity ofspectral content.

SUMMARY

A light concentrating optic for use with a photovoltaic device includesa light pipe having a first end portion for receiving light and a secondend portion for outputting concentrated light. An optical element iscoupled to the light pipe on the first end portion and configured toform an optical interface between the light pipe and the opticalelement. The optical element includes at least one light transmittingsurface configured to redirect incident light entering the opticalelement to disperse the light to fall incident on side walls of thelight pipe to increase homogeneity and intensity of light through thesecond end portion.

Another light concentrating optic for use with a photovoltaic deviceincludes a faceted light pipe having a first end portion for receivinglight and a second end portion for outputting concentrated light. Anoptical element has at least one light transmitting surface configuredto redirect incident light entering the optical element to disperse thelight to fall incident on side walls of the light pipe to increasehomogeneity and intensity of light through the second end portion. Atransmissive interface medium is configured to couple light from theoptical element to the light pipe. The transmissive interface is alsoconfigured to permit transmission of light between the optical elementand the light pipe to form an optically diffusive interfacetherebetween.

A method for fabricating a light concentrating optic includes mountingan optical element on a first end portion of a light pipe for receivinglight, with a second end portion of the light pipe for outputtingconcentrated light; and bonding the optical element to the light pipe onthe first end portion with a transmissive medium to form an opticallytransmissive interface between the light pipe and the optical element,the optical element having at least one light transmitting surfaceconfigured to redirect incident light entering the optical element todisperse the light to fall incident on side walls of the light pipe toincrease homogeneity and intensity of light through the second endportion.

Another method for concentrating light for a photovoltaic deviceincludes molding an optical element to form at least one lighttransmitting surface configured to redirect incident light entering theoptical element; mounting the optical element on a first end portion ofa light pipe, the first end portion for receiving light, and a secondend portion of the light pipe for outputting concentrated light; andforming an optically transmissive interface between the optical elementand the light pipe by employing a transmissive medium between the lightpipe and the optical element, such that light redirected by the opticalelement falls incident on side walls of the light pipe to increasehomogeneity and intensity of light through the second end portion.

These and other features and advantages will become apparent from thefollowing detailed description of illustrative embodiments thereof,which is to be read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

The disclosure will provide details in the following description ofpreferred embodiments with reference to the following figures wherein:

FIG. 1 is a diagram illustrating an improved secondary optic using adiffusive element, in accordance with one embodiment;

FIG. 2 is a diagram illustrating an improved secondary optic using auniform array element, in accordance with one embodiment;

FIG. 3 is a top view of a lenslet array which may be employed in thestructure of FIG. 2 in accordance with one embodiment;

FIG. 4 is a top view of a linear diffuser which may be employed in thestructure of FIG. 2 in accordance with one embodiment;

FIG. 5 is a diagram illustrating an improved secondary optic utilizing aplano convex element in accordance with one embodiment;

FIG. 6 is a diagram illustrating an improved secondary optic utilizing aplano convex element and a diffusive element in accordance with oneembodiment;

FIG. 7 is a diagram illustrating an improved secondary optic utilizing aplano convex element and a diffusive element integrally formed togetherin accordance with one embodiment; and

FIG. 8 is a block/flow diagram showing methods for fabricating a lightconcentrating optic in accordance with illustrative embodiments.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In accordance with the present principles, an improved light pipe devicefor solar applications is provided that combines diffusive and/ordiffractive structures with concentrating light pipes. A secondary opticis located at an entrance surface of a light pipe or directly moldedinto the light pipe to improve spatial and spectral homogenization anduniformity of light transmitted to a photovoltaic cell. The presentprinciples may be employed in many light or radiation directingapplications and are particularly useful when employed with ConcentratorPhoto-Voltaic (CPV) systems.

Methods, systems and devices, in accordance with the present principles,provide light pipe improvements to more efficiently homogenize anoptical spectrum and intensity of incident solar radiation imaged intothe light pipe. The homogenized and intensified light is transmitted toa photovoltaic cell with a negligible loss of power by incorporating andattaching at least one of a convex lens, a diffuser, a combination of aconvex lens and a diffuser, etc. to the top of the light pipe using analmost transparent material interface. It should be understood that anumber of different light refracting, reflecting and redirectingstructures or combinations thereof may be employed in accordance withthe present principles.

A common form of light pipe used in solar concentrators comprises aglass hexahedron wherein a top and bottom surfaces are parallel andinclude orthogonal square surfaces. Light enters the top surface andexits the bottom surface. The top surface comprises a square area thatis greater than or equal to the bottom surface. The side surfacescomprise four isosceles trapezoids. In accordance with the presentprinciples, light is transmitted through the top surface and the bottomsurface with minimal reflective losses. It is intended that light withinthe light pipe that is incident on side surfaces be reflected by theside surfaces efficiently via total internal reflection. Alternateembodiments apply reflective coatings to the side surfaces andantireflective coatings to the top and/or bottom surfaces. An alternateembodiment comprises a light pipe comprised of mirrored sides only andis referred to as a light cup. Alternate embodiments of the solid lightpipe comprise materials including at least one of plastic, quartz, andsapphire. However, conventional light pipes are subject to imagingartifacts leading to nonhomogeneous light intensity and spectral contentat the photovoltaic surface. The present principles eliminate asignificant amount of this problem by modifying the incident light byproviding an optic element or modification to the entrance surface of aconventional light pipe.

To improve light homogeneity on a conventional hexahedral prism typelight pipe (or other faceted structure), the provided optic homogenizesthree spectral regions. Spatial homogeneity is an important aspectbecause each area of the cell needs to be illuminated equally by thesolar rays in the three spectral regions to have a most efficient powerconversion. In one embodiment, a relatively short light pipe may haveaffixed to the top thereof one of a convex lens, a diffuser or acombination of a convex lens and a diffuser. The incorporation of suchoptics causes light incident on the entrance surface to undergo anincreased number of internal reflections within the media, which helpsrandomize the three optical intensities over the cell area and therebyhomogenize the three spectral regions. In the case of solarapplications, the three spectral regions may include wavelengths between350 and 660 nm, between 660 and 860 nm, and between 860 and 1600 nm.

A further benefit to the additional optics includes enhancing anacceptance angle of the optic. The solar concentrating system can acceptincident sunlight from a larger range of off axis angles, which providesthe system with larger error tolerance in directing the concentrator atthe sun.

It is to be understood that the present invention will be described interms of a given illustrative architecture having optical components andarrangements for photovoltaic systems; however, other architectures,structures, materials and process features and steps may be variedwithin the scope of the present invention.

It will also be understood that when an element such as a component orregion is referred to as being “on” or “over” another element, it can bedirectly on the other element or intervening elements may also bepresent. In contrast, when an element is referred to as being “directlyon” or “directly over” another element, there are no interveningelements present. It will also be understood that when an element isreferred to as being “connected” or “coupled” to another element, it canbe directly connected or coupled to the other element or interveningelements may be present. In contrast, when an element is referred to asbeing “directly connected” or “directly coupled” to another element,there are no intervening elements present.

A design for a photovoltaic device may be created for integrated circuitintegration or may be combined with components on a printed circuitboard. Methods as described herein may be used in the fabrication ofphotovoltaic devices. The devices may be integrated with other chips,discrete circuit elements, and/or other signal processing devices aspart of either (a) an intermediate product, such as a solar collectioncomponent, or (b) an end product, such as a solar collection system. Theintermediate or end products can be any product that ranges from toys,energy collectors, solar devices, light sensitive devices and otherapplications including computer products or devices having a display, akeyboard or other input device, and a central processor. Thephotovoltaic devices described herein are particularly useful for solarcells or panels employed to provide power for power utilities as well asfor electronic devices, homes, buildings, vehicles, etc.

Reference in the specification to “one embodiment” or “an embodiment” ofthe present principles, as well as other variations thereof, means thata particular feature, structure, characteristic, and so forth describedin connection with the embodiment is included in at least one embodimentof the present principles. Thus, the appearances of the phrase “in oneembodiment” or “in an embodiment”, as well any other variations,appearing in various places throughout the specification are notnecessarily all referring to the same embodiment.

It is to be appreciated that the use of any of the following “/”,“and/or”, and “at least one of” for example, in the cases of “A/B”, “Aand/or B” and “at least one of A and B”, is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of both options (A andB). As a further example, in the cases of “A, B, and/or C” and “at leastone of A, B, and C”, such phrasing is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of the third listedoption (C) only, or the selection of the first and the second listedoptions (A and B) only, or the selection of the first and third listedoptions (A and C) only, or the selection of the second and third listedoptions (B and C) only, or the selection of all three options (A and Band C). This may be extended, as readily apparent by one of ordinaryskill in this and related arts, for as many items listed.

Referring now to the drawings in which like numerals represent the sameor similar elements and initially to FIG. 1, a light focusing system 100is shown in accordance with one illustrative embodiment. The system 100includes a light pipe 101 and a diffuser 102, which is affixed to thelight pipe 101 by an interface material 103. The light pipe 101 mayinclude a conventional light pipe modified in accordance with thepresent principles. The light pipe 101 includes an optical index ofrefraction, n1. The diffuser 102 includes an optical index ofrefraction, n2. The interface medium 103 includes an optical index ofrefraction, n3. The optical indices of these components, respectively,n1, n2, n3, are chosen to be close in value, e.g., within about 10% ofeach other. Also, n2 and n3 may have different values to permitdiffusion of light at an interface between the diffuser 102 and theinterface material 103. Values for the indices may be between about 1.3and about 1.5, although other index values are also contemplated. In aparticularly useful embodiment, the indices have a difference of between0.1 and 0.01. In one embodiment, the interface material 103 providesadhesion between diffuser 102 and light pipe 101.

The diffuser 102 serves to spread focused and incident light 104 byadding a random directional component to the light 104. This can occurwithin the diffuser 102, but more generally, at one of the surfaces ofthe diffuser 102, which has been configured to be non-flat. An angle ofthe diffuser's surface preferably changes locally. By design, a range oflocal surface angles can be chosen to vary between zero for a flat localsurface area to a maximum steep local angle, e.g., between about 10 to60 degrees.

As a result of diffusion, many light rays are forced to reflect offsides 108 of the light pipe 101 before reaching an exit surface 109 atthe bottom end of the light pipe 101. This mixes and randomizes the raysof diffused light 105 thereby homogenizing the light intensities overthe exit surface 109 of the light pipe 101. In one embodiment, the sidesor sidewalls 108 include a reflective material, such as a paint or ametal, e.g., Cr, Al, Ag, etc. Antireflective coatings may be applied toa top and/or bottom surface of the light pipe or any other surface ofthe system 100 or other systems described herein.

A photovoltaic device 110 is placed or mounted adjacent to the exitsurface 109 to receive the homogenized and intensified light from thelight pipe 101. The photovoltaic device 110 may include any type ofsolar cell or light sensitive device, including single-junction cells,multi-cell junction (tandem) cells, etc. The junctions as employed hererefer to semiconductor junctions.

The improvement in homogeneity is a function of the optical indices n1to n3 and of the diffuser angles. Shallow angles (e.g., 10 to 30degrees) lead to lower homogenization and an improvement of a fewpercent in cell output. However, large angles (e.g., 45 to 60 degrees)increase homogenization, but lead to light escaping from the secondaryoptics when the rays are too steep to undergo total internal reflection.For each design in accordance with the present principles, there areoptimal values of optical indices n1 to n3 and of diffuser angles thatmaximize homogenization without substantial light leakage. These valuesmay be determined experimentally and/or using optical ray tracingcomputer modeling.

Referring to FIG. 2, another light focusing system 200 is illustrativelyshown. The system 200 includes a light pipe 201, a diffuser 202 and aninterface medium 203 disposed therebetween. The light pipe 201 mayinclude a conventional light pipe modified in accordance with thepresent principles. As before, the light pipe 201 includes an opticalindex of refraction, n1. The diffuser 202 includes an optical index ofrefraction, n2. The interface medium 203 includes an optical index ofrefraction, n3. The optical indices of these components, respectively,n1, n2, n3, are chosen to be close in value, e.g., within about 10% ofeach other. Also, n2 and n3 may have different values to permitdiffusion of light at an interface between the diffuser 202 and theinterface material 203. Values for the indices may be between about 1.3and about 1.5, although other index values are also contemplated. In aparticularly useful embodiment, the indices have a difference of between0.1 and 0.01. In one embodiment the interface material 203 providesadhesion between diffuser 202 and light pipe 201.

The diffuser 202 includes a more uniform structure than the diffuser 102of FIG. 1. In one embodiment, the diffuser 202 comprises a lenslet arraysheet or a holographic surface wherein each small lens or surfacespreads the light into the light pipe 201. An output beam 210 is asuperposition of many smaller beams either directly or indirectly ofdiffused light 205. For this purpose, the ideal lenslet array is devoidof sharp corner features that diffuse incident light 204 in alldirections.

While the diffuser 202 may include a lenslet array and/or a holographicsurface, the diffuser may also include a linear or non-linear wavysurface. The lenslet array, or linear/non-linear wavy surface may beformed from an embossed plastic material. In other embodiments, thediffuser may include a diffractive structure, which may include gratinglines or slits or include one or more convex lenses. In otherembodiments, combinations of elements may be employed, e.g., acombination of convex lenses and wavy surfaces, etc.

Referring to FIG. 3, an illustrative top view of an illustrative lensletarray 220 for use as a diffuser 202 is illustratively shown. The array220 is comprised of smooth sinusoidal or faceted surfaces 224 for eachsmall lens 222. The lenses 222 spread light into the light pipe 201 anddiffuse the light in cones with angles, e.g., between about 10 and 60degrees.

Referring to FIG. 4 with continued reference to FIG. 2, an illustrativetop view of a linear diffuser 302 for use as a diffuser 202 isillustratively shown. The linear diffuser 302 includes wave-likestructured lines 304. When attaching the diffuser 220 to the light pipe201 using the transparent/transmissive interface 203, the lines 304 ofthe linear diffuser need to be perpendicular to sun movement. This willspread the light 204 forming lines of refracted light (diffused light205) which will then be reflected off the sides of the light pipe 201towards the exit end as a homogenized beam. In the embodiment depictedin FIG. 4, an advantage is provided primarily in a single axis of tilt.It should be understood that a second axis of tilt can also beaccommodated by providing additional lines perpendicular to the sun'smovement in a second tilt axis.

Referring to FIG. 5, another light focusing system 400 is illustrativelyshown. The system 400 includes a light pipe 401, a lens 402 and aninterface medium 403 disposed therebetween. The light pipe 401 mayinclude a conventional light pipe modified in accordance with thepresent principles. As before, the light pipe 401 includes an opticalindex of refraction, n1. The lens 402 includes an optical index ofrefraction, n2. The interface medium 403 includes an optical index ofrefraction, n3. The optical indices of these components, respectively,n1, n2, n3, are chosen to be close in value, e.g., within about 10% ofeach other. Values for the indices may be between about 1.3 and about1.5, although other index values are also contemplated. In aparticularly useful embodiment, the indices have a difference of between0.1 and 0.01. In one embodiment the interface material 403 providesadhesion between lens 402 and light pipe 401.

In one embodiment, the lens 402 includes a convex (convergent) lens,which is affixed to the light pipe 401, using the interface material403. The lens 402 further bends rays of incident light 404 at angles asre-imaged light 405, which meets at a central region 410, or a smallfocal image, and then spreads to be reflected off sides 412 of the lightpipe 401. The lens 402 may include glass, a mineral, a plastic or othersuitable light transmissive material. The lens 402 may also include amulti-element lens. The convex lens 402 may include, e.g., a sphericalprofile, an aspheric profile, a parabolic profile, etc. In oneembodiment, lens 402 may include a doublet structure.

By employing the lens 402 (and/or the diffusers of the otherembodiments), the angular content of rays entering the light pipe 401 isincreased. Note that in a concentrating light pipe or light guide, thereis a limit to the amount of angle relative to a central axis that can beuseful. In the case of light pipes that reflect light incident on theside walls by total internal reflection, the incident angle needs to berestricted to below the critical angle relative to the side surface(412). The critical angle is determined by the materials of the lightpipe 401 and its surroundings. Above the critical angle, efficiency islost due to light leakage out of the sides. Another consideration inconcentrating light pipes with mirrored side walls is that if the anglerelative to a tapered side (412) exceeds 90 degrees, the light willretro reflect out of the top of the light pipe. This restriction alsolimits the taper angle of the light pipe and its length.

In one embodiment, for a system optimized for 1600 times concentration,the lens 402 is optimal at approximately a 40 mm focal length. The beamat an exit face 414 is further randomized and homogenized due to theincrease in angular content, and an increased number of reflectionsagainst the sides 412. A further advantage of the embodiments shown forthe convex lens 402 includes an improvement to an acceptance angle 416of the light pipe 401. In this way, incident light 404 may be receivedfrom wider angles. Note that the acceptance angle (416) may be increasedfor all embodiments in accordance with the present principles.

System 400 (or other systems as described herein) may be produced bymolding a convex surface, etc. comprising optical plastic or liquidglass directly on a light pipe surface. For example, a liquid glass orplastic heated above the liquid transition temperature may be poured ina suitably designed mold, having a semi-spherical or convex shape.Instead of attaching a convex lens, a molded light pipe is morereliable, sustainable and eliminates an interface (403). Alternately,the optic (lens) may include other moldable or curable materials suchas, e.g., at least one of a plastic, a thermoplastic, a UV curedplastic, a catalyst cured plastic, a glass, etc. The light pipe 401 orlens 402 may provide a light transmitting surface such as a convex shapeusing, e.g., a compression molding technique, injection moldedtechnique, etc. The molded materials may include fluoropolymer, acrylic,epoxy, silicone, etc.

It should be understood that this and other embodiments may be formed asa unitary device including all components in a same monolithiccomponent. For example, the light pipe 401 and the convex lens 402 maybe formed in a same molding process. In addition, a light pipe may bemodified to include a diffuser structure thereon by a machining or otherprocess.

Referring to FIG. 6, in another embodiment, a focusing system 500includes a combination of a convex lens 502 and a diffuser 504 affixedto each other and a light pipe 501 using interface materials 503 and505. This combination gives more optimum randomization andhomogenization for incident light 506 as well as enhancing theacceptance angle.

The light pipe 501 may include a conventional light pipe modified inaccordance with the present principles. The light pipe 501 includes anoptical index of refraction, n1. The lens 502 includes an optical indexof refraction, n2. The interface medium 503 includes an optical index ofrefraction, n3. The diffuser 504 includes an optical index ofrefraction, n4. The interface medium 505 includes an optical index ofrefraction, n5. The optical indices of these components, respectively,n1, n2, n3, n4, n5 are chosen to be close in value, e.g., within about10% of each other. Values for the indices may be between about 1.3 andabout 1.5, although other index values are also contemplated. In aparticularly useful embodiment, the indices have a difference of between0.1 and 0.01. In one embodiment the interface materials 503 and 505provide adhesion between lens 502, diffuser 504 and light pipe 501.

It should be understood that the diffuser 504 may include uniform ornon-uniform surfaces for diffusing incident light. For example, thediffuser 504 may include lenslets, linear diffuser, sinusoidal shapes,varying surfaces, etc. The interface medium 503 and/or 505 may include asame material. Likewise, the optical element (diffuser lens, etc.) mayinclude a same material as the light pipe and even as the interfacemedium, which may include, e.g., a substantially transparent adhesive,an index matching fluid, an acrylic, an epoxy, a silicone, Krytox™grease, etc.

Referring to FIG. 7, in a focusing system 600, a combination opticalelement 602 includes a convex lens and a diffusing surface in a singleelement. The element 602 includes a convex top surface 610, and adiffusing bottom surface 612. The diffusing bottom surface 612 mayinclude at least one of a diffuser, a lenslet array, a linear diffuser,etc. Advantages of this design are simplicity and cost, particularlysince element 602 can be mass produced by cost-effective processes suchas molding.

The element 602 is affixed to a light pipe 601 using interface materials603. The light pipe 601 may include a conventional light pipe modifiedin accordance with the present principles. The indexes of refraction aresimilar as described with reference to FIG. 5. A plano convex lensprofile (e.g., for lenses 402, 502 and element 602) includes but is notlimited to at least one of a spherical profile and a parabolic profile.

Embodiments described herein may be fabricated by combining the lightpipe (101, 201, 401, 501, 601) with another optical apparatus (e.g.,102, 202, 402, 502, 602) through an interface (e.g., 103, 203, 403, 503,603). In one embodiment, the interface is transparent or nearlytransparent (transmissive). In other embodiments, the interface providessome diffusion of light passing through it. The interfaces describedherein may include a silicon adhesive. In another embodiment, thetransparent interface may include Krytox™ grease. Other adhesives ormaterials may also be employed. For example, interface materials in thepresent embodiments include but are not limited to at least one of anoptical adhesive, an index matching fluid, an epoxy based compound, asilicone based compound, an acrylic based compound, etc. Oneconsideration in selecting an optical interface material is toefficiently couple light from one section of the optic to the next.Index matching for transparency may also be implemented.

Referring to FIG. 8, methods for concentrating light are illustrativelyshown in accordance with the present principles. It should also be notedthat, in some alternative implementations, the functions noted in theblocks may occur out of the order noted in the figures. For example, twoblocks shown in succession may, in fact, be executed substantiallyconcurrently, or the blocks may sometimes be executed in the reverseorder, depending upon the functionality involved. It will also be notedthat each block of the block diagrams and/or flowchart illustration, andcombinations of blocks in the block diagrams and/or flowchartillustration, can be implemented by special purpose hardware-basedsystems that perform the specified functions or acts, or combinations ofspecial purpose hardware and computer instructions.

In block 702, an optical element is fabricated or configured. Theoptical element may be formed on a conventional light pipe by machininga lens or diffuser in a light receiving end. The optical element may bemolded using a molding technique such as injection molding, compressionmolding or other molding process. The optical element may be configuredwith one or more transmission surfaces, which may include lenses, arraysof shapes, etc. The light transmitting surface may include at least oneof a light diffusive surface, a light diffusive pattern, a lightdiffractive or a light refractive structure, at least one of a lensletarray, a holographic surface, a linear diffuser, non-linear wavysurface, a convex lens, or any combination thereof. For example, theoptical element may include a convex lens integrally molded with adiffuser.

In block 704, if the optical element is separate from the light pipe,the optical element, formed by whatever method, is mounted on a firstend portion of a light pipe for receiving light. A second end portion ofthe light pipe outputs concentrated light. The light pipe may include anumber of geometric configurations including a hexahedral, a conicalshape, a cylindrical shape, or other prisms or magnification geometry.

The optical element includes the at least one light transmittingsurface, which is configured to redirect incident light entering theoptical element by dispersing the light to fall incident on side wallsof the light pipe to increase homogeneity and intensity of light throughthe second end portion.

In block 706, the optical element is bonded to the light pipe on thefirst end portion in one embodiment. In block 708, bonding may includeapplying at least one of an acrylic, an epoxy, a silicone or grease,although other materials are also contemplated. The bonding may includeusing an index matched transmissive medium to form an opticallytransparent interface between the light pipe and the optical element. Inother embodiments, differences between the optical indexes of the lightpipe and the interface material (or other components) are employed tohelp diffusion. In one embodiment, the optical element, opticalinterface material and the light pipe include different optical indiceswithin a range of 0.01 to 0.1 of each other.

In block 712, the optical element is configured to confine incidentlight within the light pipe by total internal reflection from thesidewalls. In block 714, the sidewalls of the light pipe may be coatedwith a reflective material to improve internal reflections.

In block 716, the second end portion of the light pipe is affixed to orheld adjacent to a photovoltaic device. The light pipe concentrates thelight received to homogenously apply the intensified light across thephotovoltaic device. A plurality of optical elements, light pipes andphotovoltaic devices may be arranged in an array or other configurationfor converting radiation into electrical energy.

Having described preferred embodiments of a secondary optic forconcentrating solar photovoltaic (which are intended to be illustrativeand not limiting), it is noted that modifications and variations can bemade by persons skilled in the art in light of the above teachings. Itis therefore to be understood that changes may be made in the particularembodiments disclosed which are within the scope of the invention asoutlined by the appended claims. Having thus described aspects of theinvention, with the details and particularity required by the patentlaws, what is claimed and desired protected by Letters Patent is setforth in the appended claims.

What is claimed is:
 1. A light concentrating optic configured for usewith a photovoltaic device, comprising: a light pipe having a first endportion for receiving light and a second end portion for outputtingconcentrated light; a first optical interface material being an adhesiveconfigured to provide adhesion between an optical element and the lightpipe, the adhesive extending circumferentially along the light pipe; theoptical element configured to form an optical interface between thelight pipe and the optical element, the optical element having at leastone light transmitting surface facing toward the first end portion ofthe light pipe that is configured to redirect incident light enteringthe optical element to disperse the light to fall incident on side wallsof the light pipe to increase homogeneity and intensity of light throughthe second end portion; and a convex lens affixed to the optical elementby a second optical interface material, wherein the second opticalinterface material has a different index of refraction than the firstoptical interface material.
 2. The optic as recited in claim 1, whereinthe at least one light transmitting surface with at least one of a lightdiffusive surface or light diffusive pattern.
 3. The optic as recited inclaim 1, wherein the optical interface includes at least one lighttransmitting surface that includes at least one of a light diffractiveor a light refractive structure.
 4. The optic as recited in claim 1,wherein the light pipe has an index of refraction different than theindex of refraction of both the first and second optical interfacematerials.
 5. The optic as recited in claim 1, wherein the opticalelement has an index of refraction different than the index ofrefraction of both the first and second optical interface materials. 6.The optic as recited in claim 1, wherein the optical element isconfigured to confine incident light within the light pipe by totalinternal reflection from the sidewalls.
 7. The optic as recited in claim1, wherein light entering the light pipe is confined by means ofreflection from the side walls that are coated with a reflectivematerial.
 8. The optic as recited in claim 1, wherein the opticalelement includes at least one of a lenslet array, a holographic surface,a linear diffuser, non-linear wavy surface, a convex lens, or acombination thereof.
 9. The optic as recited in claim 1, wherein theoptical element includes a convex lens integrally formed with adiffuser.
 10. The optic as recited in claim 1, wherein the first opticalinterface material includes a silicon adhesive and the second opticalinterface material includes an index matching fluid.
 11. A lightconcentrating optic configured for use with a photovoltaic device,comprising: a faceted light pipe having a first end portion forreceiving light and a second end portion for outputting concentratedlight; a first transmissive interface medium being an adhesiveconfigured to provide adhesion between an optical element and the lightpipe, the adhesive extending circumferentially along the faceted lightpipe and the first transmissive interface medium being configured topermit transmission of light between the optical element and the lightpipe to form an optically diffusive interface therebetween; the opticalelement having at least one light transmitting surface facing toward thefirst end portion of the light pipe configured to redirect incidentlight entering the optical element to disperse the light to fallincident on side walls of the light pipe to increase homogeneity andintensity of light through the second end portion; and a convex lensaffixed to the optical element by a second transmissive interfacemedium, wherein the second transmissive interface medium has a differentindex of refraction than the first transmissive interface medium. 12.The optic as recited in claim 11, wherein the light pipe has an index ofrefraction different than the index of refraction of both the first andsecond transmissive interface mediums.
 13. The optic as recited in claim11, wherein the first transmissive interface medium includes a siliconadhesive and the second transmissive interface medium includes an indexmatching fluid.